It is generally desireable for liquid retaining reservoirs, such as used in conjunction with many bearings, to have a filler opening which can be easily opened for the addition of lubricant or other media.
In the past, closures which have been used to seal filler openings have been plagued with a series of problems. Many have been easily damaged. The so-called "self-closing" types have not provided good seals. The twist or screw type closures often times are left open because a worker forgets to re-apply the closure or the screw closures are cocked on re-application resulting in leakage.